Seed plants were the most recent evolutionary step

The big picture: We now live in a seed plant world


  • Seeds changed the course of plant evolution
    • Gymnosperms evolve ~360 mya
    • drier climate = new selection pressure
    • Elkensia first fossil seed plant


  • Seeds consist of embryo & nutrients
    • surrounded by protective coat
    • seeds can be dispersed long distances
    • supply of food for embryo


  • 2 living seed lineages
    • gymnosperms ( naked seed)
    • angiosperms ( covered seed)

Why seeds matter so much…


  • Before seeds, spore was only protective structure
    • allowed long-distance dispersal but…
    • dependent young embryo still left exposed


  • Seeds include multi-cellular endospore = food tissue


  • Embryos can remain dormant for a long time inside seed coat


  • Gymnosperms: seeds exposed on sporophylls, form cones
  • Angiosperms: seeds enclosed inside ovary of fruit

Seed plants further adapt to life on land

  • Miniaturization/protection of gametophyte
    • mosses → ferns → seed plants
    • gametophyte became microscopic


  • Heterosporous life cycle only
    • 2 kinds of spores
    • form male or female gametopytes
    • more resources allocated to female spores


  • Ovules (protect female gametophyte)
    • ovule contains female spores


  • Pollen (protect male gametophyte)
    • male spores develops into pollen grain

Gymnosperms take over first


  • Seed plant ancestors now extinct


  • ~300-250 mya climate become drier
    • Carboniferous → Permian
    • new gymnosperms forests


  • Many gymnosperms have thin needle-like leaves
    • reduce surface area
    • sunken stomata
    • evergreen
    • thick waxy cuticle

4 living lineages of gymnosperms

  • Cycads: 300 living species
    • earliest evidence of insect pollination
    • Cycads and dinosaurs lived together


  • Gnteophytes: small and super weird group
    • source of ephedrine


  • Ginkos: Ginkgo biloba is only species left!
    • super common street tree


  • Conifers: ~600 species (cones)
    • evergreen and deciduous
    • leaves are needles or scales

Gymnosperm reproduction: everything happens on cones


INTERNAL FERTILIZATION (Seed Plant Trait)

  • Male and female cones on same plant (mostly)
    • cone scales → modified leaves → sporophyll
    • sporophyll → sporangia → spores
    • pollen (male) and ovulate (female) cones


  • Male cones house microsporangium
    • microsporangium produces microspores
    • microspores develop into pollen grains
    • microgametophyte inside pollen grain
    • pollen dispersed by wind
    • male cones usually on bottom of tree

Gymnosperm reproduction: everything happens on cones


  • Female cones have 2 ovules per scale
    • each ovule has megasproangium
    • megasporangium produce megaspores
    • megaspore develops megagametophyte with egg


  • Pollen grain reaches ovule and germinates
    • pollen tube grows toward megaspore
    • sperm → pollen tube → egg (no water!)
    • seed forms, dispersed by wind


  • Gymnosperm reproduction requires wind pollination
    • very inefficient, so lots of pollen produced

Flowering plant diversity


  • Most diverse living group of plants
    • 14,000 genera, >250,000 species

    • 90% of plant diversity

  • 2 classes: Monocots and Dicots
    • monocots = one seed leaf in embryo
    • dicots = two seed leaf in embryo

Angiosperm evolution: pollination (plants + insects)

Insect pollination improves on inefficient wind pollination in conifers
Evolved ~125 million years ago and diversified rapidly

Angiosperm reproduction: everything happens on flowers


  • Flower is a modified stem for reproduction
    • with 4 types of modified leaves


  • Sepals: leaves at base which enclose flower
    • usually green


  • Petals: colored leaves to attract pollinators
    • less colored if wind pollinated


  • Two types of fertile organs to make spores
    • sporophylls (male and female)

Flowers - Male



  • Stamens: Male fertile organ
    • microsporophyll (male fertile leaf)
    • produce microspores that develop into pollen


  • Anther: where microsporangia develop
    • microspores produced
    • microspores → pollen
    • pollen → male gametophyte


  • Filament: stalk that holds the anther

Flowers - Female

  • Carpel: Female fertile organ
    • megasporophyll (female fertile leaf)


  • Stigma: sticky top that receives pollen


  • Style: leads from stigma to ovary
    • where pollen tube forms ( INTERNAL FERTILIZATION)


  • Ovary: contains ovules
    • megasporangium inside ovule produces megaspores
    • megaspore develops into female gametophyte w/ egg
    • after fertilization, ovary becomes ‘fruit’

Angiosperm reproduction: double fertilization

Comparing seeds of gymnosperm and angiosperms

Seed coat devlops from ovule (2N), embryo = 2N, endospore (food cells) = ???